a) Field of the Invention
The present invention relates to an eyepiece lens system, and more specifically to an eyepiece lens system having a wide visual field and favorably corrected aberrations.
b) Description of the Prior Art
Eyepiece lens systems are a kind of magnifying lenses and generally have simple compositions consisting only of convex lens elements. Most of the eyepiece lens systems of such compositions have Petzval's sums of large positive values, allow curvature of field to remain therein and produce remarkable distortion. For this reason, image surfaces of objective lens systems and those of eyepiece lens systems are inclined in the directions reverse to each other in optical systems for microscopes so as to form flat images with the optical systems as wholes, but such a measure cannot correct the inclinations of the images sufficiently. Further, in order to prevent the image surface from being inclined, there is adopted an auxiliary means for correcting the curvature of field by preventing inclination of an average image surface of the meridional image surface and the sagital image surface. As the conventional eyepiece lens system adopting this auxiliary correcting means, there is known the lens system disclosed by Japanese Unexamined Published Patent Application No. 57315/60. When this correcting means is adopted for a lens system having a large Petzval's sum, however, astigmatism is not corrected sufficiently since the meridional image surface is separated from the sagital image surface. Furthermore, eyepiece lens systems produce more remarkable distortion than objective lens systems in optical systems for microscopes. Accordingly, it is desired to correct distortion favorably in eyepiece lens systems.
Moreover, when an eyepiece lens system has a long eye relief, spherical aberration of pupil cannot be corrected sufficiently, vignetting may be caused at merginal portion of visual field and, in addition, it is difficult to favorably correct aberrations of the rays at the marginal portions of visual field, especially coma, astigmatism and lateral chromatic aberration.